I have one person on IGNORE. They were put there because their responses were overly aggressive and never interesting.
If you need to use IGNORE in order to enjoy your visits to the site, then you should go ahead and use it. It doesn't have to be some big reason either - whatever makes your time on the board more enjoyable is fair game. It could be politics, religion, or just plain attitude, if someone is consistently spoiling your fun, get rid.
That said, I would personally send a PM to someone who was an issue in order to clear the air, or clarify a point. I don't jump to Ignore. Sometimes people respond and either they, or myself, was simply having a bad day, and in one case - the abuse was continuing, so.........
We'd all do well to remember that the real people behind these accounts are likely very different away from the computer. It's just the way things are. Computers are abstract, and we build up persona's. So I don't take things here entirely seriously. I can dislike a post by someone in one thread, and wholly agree with them in another - I don't carry over one bad experience. Life is too short......
ps: There's at least one or maybe even two people I should have on IGNORE, but I've not done it.
